This invention relates to a carbodiimide crosslinking agent having good solubility or dispersibility in water, its preparation, and a coating material comprising the crosslinking agent. More particularly, the invention relates to a carbodiimide crosslinking agent which is suitable for crosslinking high molecular weight resins having a group capable of reacting with the carbodiimide group such as a carboxyl group, an amino group, or an alcoholic hydroxyl group, e.g. polyester resins, acrylic resins, polyvinyl alcohol resins and the like, and also to a process for preparing the agent. The invention also relates to a coating material comprising the carbodiimide crosslinking agent which can form coating films having good water, chemical and wear resistances and can be used in various fields of inks, coatings, sealants, adhesives, leather coatings, binders for non-woven fabrics, and the like.
The importance of film-forming methods has been well recognized in the coating industry. Especially, in film-forming techniques, it is the usual practice to add crosslinking agents to film-forming coating compositions so as to impart important characteristic properties such as hardness and solvent, water and wear resistances to the resultant film.
In this case, the conventional crosslinking agents include aziridine compounds, epoxy compounds, blocked isocyanate compounds, and oxazoline compounds. These crosslinking agents which have bi- or polyfunctional groups are mixed with organic compounds (typically high molecular weight resins) having a functional group capable of reacting with the functional group such as a carboxyl group, an amino group or an alcoholic hydroxyl group, prior to the formation of film. The resultant mixture is coated onto a substrate and cured at a given temperature to form a film. In this way, the crosslinking agent can impart good characteristic properties such as a hardness and solvent, water and wear resistances to the film.
However, the oxazoline compound and the blocked isocyanate compound have a curing temperature as high as 80 to 180.degree. C., thus causing a problem that they are not used for a substrate having a poor heat resistance and that an expensive equipment is necessary. In particular, the blocked isocyanate compound may adversely influence environments and workers because a blocking agent used to block the isocyanate group is so volatile that it is vaporized upon curing.
The aziridine compound and the epoxy compound are disadvantageous in that the storage stability of these compounds is so poor that when preserved under high temperature and high humidity conditions, the compound polymerizes and is converted to a hardened matter, and has very strong toxicity. Thus, care has to be paid in handling of these compounds.
In recent years, various types of carbodiimide compounds have been developed, and attention has been drawn to their good characteristics on use as a crosslinking agent. At present, commercially available carbodiimide compounds exhibit poor miscibility with resins to be crosslinked. Especially, the compounds are not satisfactory with respect to reactivity at low temperatures, thus presenting the problem that desirable physical properties are not imparted to a crosslinked resin.